1. Field of the Invention
The present invention generally relates to thick-film circuit components and their fabrication. More particularly, this invention relates to a thick-film resistor whose electrical width and length are precisely determined by concentric terminals using photolithography techniques, thereby avoiding the variability associated with conventional screen printed resistors.
2. Description of the Prior Art
Thick-film resistors are employed in electronic circuits to provide a wide range of resistor values. Such resistors are formed by printing, such as screen printing, a thick-film resistive paste or ink on a substrate, which may be a printed wiring board, flexible circuit, or a ceramic or silicon substrate. Thick-film inks are typically composed of an electrically resistive material dispersed in an organic vehicle or polymer matrix material. After printing, the thick-film ink is typically heated to convert the ink into a suitable film that adheres to the substrate. If a polymer thick-film ink is used, the heating step serves to dry and cure the polymer matrix material. Other thick-film inks must be sintered, or fired, during which the ink is heated to burn off the organic vehicle and fuse the remaining solid material.
The predictability and variability (or tolerance) of the electrical resistance of a thick-film resistor are dependent on the precision with which the resistor is produced, the stability of the resistor material, and the stability of the resistor terminations. Conventional thick-film resistors are rectangular shape, with "x," "y" and "z" dimensions corresponding to the electrical width (W), electrical length (L) and thickness, respectively, of the resistor. Control of the "x" and "y" dimensions of a thick-film resistor is particularly challenging in view of the techniques employed to print thick-film inks and the dimensional changes that may occur during subsequent processing. For rectangular screen-printed resistors, the x and z dimensions are defined by the resistor screening process, and the y dimension is defined by the termination pattern. Conventional screen printing techniques generally employ a template with apertures bearing the positive image of the resistor to be created. The template, referred to as a screening mask, is placed above and in close proximity to the surface of the substrate on which the resistor is to be formed. The mask is then loaded with the resistive ink, and a squeegee blade is drawn across the surface of the mask to press the ink through the apertures and onto the surface of the substrate.
Compared to many other deposition processes, screen printing is a relatively imprecise process. Screen printed thick-film resistors having adequate tolerances in the x and y dimensions are often physically larger than chip resistors. Resistance value predictability (i.e., the dependability that a 500 microns wide, 10 square resistor will have ten times the resistance of a 1000 microns wide, 1 square resistor) is generally low, and precise tolerances cannot be maintained at aspect ratios (L/W) below 0.5 squares. As a result, one ink of a given resistivity, requiring one screening, cure and associated process steps, is required for each decade of resistance value needed in a circuit design, which often necessitates the use of three to four inks to complete one circuit. This increases cost and decreases throughput. Predictability and variation of resistance values are also strongly affected by surface planarity and resistor orientation vs. print direction. While resistance tolerances can be improved by laser trimming, such an operation is usually cost prohibitive for complex circuits. As a result, screen printed thick-film resistors have found only limited application.
From the above, it can be seen that present practices involving the fabrication of thick-film resistors can necessitate a compromise between the precision of the resistance value and the physical size of the resistor. In other words, while physically smaller resistors are often preferred to yield a more compact circuit, an undesirable consequence is that resistance values are less predictable due to the dimensional variability of the resistors. In addition, multiple resistive inks, each requiring separate cure and processing steps, are required to produce a circuit having thick-film resistors whose resistance values differ by more than one decade. Accordingly, what is needed is a method for producing thick-film resistors, in which the dimensions of a resistor can be precisely defined so as to achieve more readily a desired resistance value, and by which the resistance values of thick-film resistors formed with a single ink can be accurately achieved over a range of greater than a single decade.